A general coated optical fiber has as a coated layer at the stage of its strands a primary coated layer and a buffer layer, or a buffer layer used also as the primary coated layer, and a reinforcing coated layer on the outer periphery of the buffer layer.
With respect to the reinforcing coated layer, the layer has already formed of an FRP, and when the reinforcing coated layer is made of the FRP, the mechanical characteristics of the coated optical fiber can be largely improved, it is known that an increase in its transmission loss under the using conditions upon variation in the temperature becomes smaller than a general nylon coated optical fiber.
The above-described FRP reinforcing coated layer (which will be termed "an FRP layer") is, as already known, composed of a glass reinforced fiber material disposed along the longitudinal direction of the optical fiber, and thermosetting resin immersed with the material and cured, and the FRP layer is formed in a pipe shape directly above the buffer layer.
However, the FRP layer has only one disadvantage as will be described below.
In other words, when a compression force, twisting, or bending is acted on the FRP layer formed in the pipe shape from the side face, the layer is deformed in a direction that the pipe shape is flattened so that a longitudinal crack may occur along the longitudinal direction of the reinforced fiber material.
This is caused by the buffer layer, which is soft and lack of the above deformation preventing effect, with the result that the longitudinal crack of the FRP layer is produced.
In addition, in case of the coated optical fiber which has the FRP layer, there are still rooms for improving to enhance the transmission characteristics.
A main object of this invention is to prevent the aforementioned longitudinal crack and to also secure high transmission characteristics.